This invention relates to the field of intracorporeal medical devices, and more particularly to elongated intravascular members such as guidewires for percutaneous transluminal coronary angioplasty (PTCA) and stents for maintaining body lumen patency after the body lumen has been dilated with a balloon.
In PTCA procedures a guiding catheter is percutaneously introduced into the cardiovascular system of a patient in a conventional Seldiger technique and advanced therein until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire is positioned within an inner lumen of a dilatation catheter and then both the catheter and guidewire are advanced through the guiding catheter to the distal end thereof. The guidewire is first advanced out of the distal end of the guiding catheter into the patient""s coronary vasculature until the distal end of the guidewire crosses a lesion to be dilated, then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy over the previously introduced guidewire until the balloon is properly positioned across the lesion. Once in position across the lesion, the balloon is inflated one or more times to a predetermined size with radiopaque liquid to dilate the stenosis. The balloon is then deflated so that blood flow will resume through the dilated artery and the dilatation catheter and the guidewire can be removed therefrom.
Conventional guidewires for angioplasty and other vascular procedures usually comprise an elongated core member with one or more tapered sections near its distal end and a flexible body such as a helical coil disposed about a distal portion of the core member. A shapable member, which may be the distal extremity of the core member or a separate shaping ribbon such as described in U.S. Pat. No. 5,135,503, hereby incorporated into this application by reference, extends through the flexible body and is secured to a rounded plug at the distal end of the flexible body. Torquing means are provided on the proximal end of the core member to rotate, and thereby steer, the guidewire while it is being advanced through a patient""s vascular system. The core member is typically formed of stainless steel, although core member formed of pseudoelastic NiTi alloys are described in the literature and have been used to a limited extent in clinical applications.
Further details of guidewires, and devices associated therewith for angioplasty procedures can be found in U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson et al.); U.S. Pat. No. 4,554,929 (Samson et al.); and copending application Ser. No. 07/994,679 (Abrams et al.) which are incorporated into this application by reference.
Steerable dilatation catheters with fixed, built-in guidewires or guiding members, such as described in U.S. Pat. No. 4,582,181 (now Re 33,166) are frequently used because they have better pushability than over-the-wire dilatation catheters where the guidewires are slidably disposed within the guidewire lumens of the catheters.
A major requirement for guidewires and other guiding members is that they have sufficient column strength to be pushed through a patient""s vascular system or other body lumen without kinking. However, they must also be flexible enough to avoid damaging the blood vessel or other body lumen through which they are advanced. Efforts have been made to improve both the strength and flexibility of guidewires in order to make them more suitable for their intended uses, but these two properties can be diametrically opposed to one another in that an increase in one usually involves a decrease in the other. Efforts to combine a separate relatively stiff proximal section with a relatively flexible distal section frequently result in an abrupt transition at the junction of the proximal and distal section due to material differences.
What has been needed and heretofore unavailable is an elongated intravascular body, such as a guidewire, a stent or the like, which exhibits much higher strength coupled with good ductility than materials currently used to form these types of intravascular devices.
The present invention is directed to a high strength alloy containing cobalt, nickel, and chromium and particularly to a composite product having a portion formed of the high strength cobalt-nickel-chromium alloy and a portion formed of pseudoelastic alloy such as NiTi alloy.
The product of one embodiment of the invention is an elongated member configured for advancement within a body lumen and is formed at least in part, of high strength alloy comprising about 28 to about 65% cobalt, about 2 to about 40% nickel, about 5 to about 35% chromium and up to about 12% molybdenum. Other alloying components include up to about 20% tungsten, up to about 20% iron and up to about 3% manganese. The alloy may also contain inconsequential amounts of other alloying constituents, as well as impurities, typically less than 0.5% each. A presently preferred alloy composition for use in the intracorporeal product consists essentially of about 30 to about 45% cobalt, about 25 to about 37% nickel, about 15 to about 25% chromium and about 5 to about 15% molybdenum. As used herein all references to percent composition are weight percent unless otherwise noted. The high strength alloy has ultimate strengths up to and exceeding 300 ksi.
Preferably, the intracorporeal product is formed by first cold working the high strength alloy at least 40% of its original transverse cross-sectional area in a plurality of cold working stages with the cold worked product being intermediate annealed between cold working stages at a temperature between about 600xc2x0 and 1200xc2x0 C. Those alloys containing molydenum are age hardenable after cold working and annealing at a temperature between about 400xc2x0 and about 700xc2x0 C. For optimum tensile strength properties the aging is conducted at about 550xc2x0 to about 680xc2x0 C., particularly when the high strength alloy is combined with other alloys as described hereinafter.
In another embodiment of the invention, the cobalt-nickel-chromium alloy is formed into a composite structure with a NiTi alloy which contains about 25 to about 47% titanium and the balance nickel and up to 10% of one or more additional alloying elements. Such other alloying elements may be selected from the group consisting of up to 3% each of iron, cobalt, platinum, palladium and chromium and up to about 10% copper and vanadium. This alloy preferably has a stable austenite phase at body temperature (about 37xc2x0 C.) and exhibits pseudoelasticity with a stressed induced transformation of the austenite phase to a martensite phase at body temperature at a stress level well above about 50 ksi, preferably above 70 ksi and in many cases above about 90 ksi. The stress levels causing the complete stress-induced transformation of the austenite phase to the martensite phase results in a strain in the specimen of at least about 4%, preferably over 5%. The region of phase transformation resulting from stress preferably begins when the specimen has been strained about 1 to 2% at the onset of the phase change from austenite to martensite and extends to about 7 to about 9% strain at the completion of the phase change. The stress and strain referred to herein is measured by tensile testing. Other methods for determining the stress-strain relationship, e.g., applying a bending moment to a cantilevered specimen, provide a different relationship from the relationship determined by tensile testing, because the stresses which occur in the specimen during bending are not as uniform as they are in tensile testing. The rate of change in stress during the phase transformation is considerably less than the rate of change thereof either before or after the stress-induced transformation. The stress level is relatively constant within the transformation period.
To form the elongated pseudoelastic NiTi member, the alloy material is first cold worked in a plurality of stages, preferably by drawing, to effect a size reduction of at least about 30% and up to about 70% or more in the original transverse cross section thereof with intermediate annealing between the cold working stages at temperatures between about 600xc2x0 to about 800xc2x0 C. for about 5 to about 30 minutes. After the final cold working stage the cold worked product is given a final anneal at a temperature of about 700xc2x0 C. to generate final properties. Preferably, the cold worked NiTi alloy product is subjected to tension during the final annealing and/or provided with a mechanical straightening followed by thermal treatment to help develop a straight memory. The ultimate tensile strength of the material is well above 200 ksi with an ultimate elongation at failure of about 15%.
In one aspect of the invention the cobalt-nickel-chromium containing alloy and another alloy such as the NiTi alloy described above are cold worked together into a composite product, with both alloys being subjected to the same thermomechanical processing to develop a desirable combination of properties. In particular, a presently preferred thermomechanical processing includes a plurality of drawing steps with a reduction of at least about 25% in each cold working stage. The cold worked product is intermediate annealed between cold working stages at a temperature of about 600xc2x0 and 900xc2x0 C., e.g. about 750xc2x0 C. with a time at temperature of about 10 to about 15 minutes. The amount of cold work in the last working stage should be at least about 50% and can be as high as 95% or more. However, the actual cold working in the final working stage is usually determined by the elongation or ductility desired in the final product after straightening and aging.
In the above embodiment the elongated Nixe2x80x94Ti alloy product is an inner member disposed within the inner lumen of an elongated sheath formed of a Coxe2x80x94Nixe2x80x94Crxe2x80x94Mo alloy with an appropriate lubricant and then the assembled unit is processed in a series of size reduction steps involving drawing, or other cold working, followed by an intermediate annealing as described above. The annealing may be performed in line with the drawing. The Coxe2x80x94Nixe2x80x94Crxe2x80x94Mo alloy sheath and the NiTi alloy inner member should be recrystallization annealed prior to assembly and cold work to provide maximum ductility by maintaining an equiaxed grain structure and minimum grain growth. After the final cold working step, the composite product is heat treated at a temperature between about 500xc2x0 and 700xc2x0 C. and preferably between about 550xc2x0 and 675xc2x0 C. for about one minute to about four hours to age harden the cladding and provide pseudoelastic characteristics to the inner member. Tension may be applied during the aging treatment to straighten the product while it is being aged and to provide a straight memory to the NiTi alloy portion of the composite. For composite products with an inner member formed of alloys other than Nixe2x80x94Ti alloys, the aging conditions, i.e. the temperature and the time at temperature, may be different than that described above for NiTi alloys.
In an alternative embodiment, the NiTi alloy product and the Coxe2x80x94Nixe2x80x94Cr alloy product can be first prepared separately to their desired final properties and then combined together by suitable means to form the composite product. For example, after final processing, the Coxe2x80x94Nixe2x80x94Cr alloy sheath can be heated to expand the inner lumen therein so that an NiTi inner member can be readily inserted therein. After insertion of the NiTi inner member into the inner lumen of the sheath, the latter can be cooled so that it shrink fits about the NiTi inner member. Alternatively, the NiTi inner member can be inserted into the sheath after processing while the sheath is still at elevated temperatures and then cooled to contract the sheath onto the NiTi inner member. Other means for combining the NiTi product and the Coxe2x80x94Nixe2x80x94Cr product includes the use of an adhesive bond therebetween or a physical connection such as a set screw extending through the sheath into the core member or some other type of mechanical connection. A wide variety of other means for joining the Nixe2x80x94Ti product and the Coxe2x80x94Nixe2x80x94Cr are contemplated and will become apparent to those skilled in the art.
The products of the invention exhibit a very high level of tensile strength, yet they have excellent ductility. The age hardened Coxe2x80x94Nixe2x80x94Crxe2x80x94Mo alloy can have ultimate tensile strengths above 300 ksi and the NiTi alloys can have ultimate tensile strengths exceeding 200 ksi. These products are biocompatible and are particularly useful in medical devices, which are to be utilized intracorporeally, such as guidewires, stents and the like.
One embodiment of the present invention is directed to an expandable stent which is relatively flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but which is stiff and stable enough radially in an expanded condition to maintain the patency of a body lumen such as an artery when implanted therein.
The stent of the invention generally includes a plurality of radially expandable cylindrical elements which are relatively independent in their ability to expand and to flex relative to one another. The individual radially expandable cylindrical elements of the stent are dimensioned so as to be longitudinally shorter than their own diameters. Interconnecting elements or struts extending between adjacent cylindrical elements provide increased stability and are preferably positioned to prevent warping of the stent upon the expansion thereof. The resulting stent structure is a series of radially expandable cylindrical elements which are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent. The individual cylindrical elements may rotate slightly relative to adjacent cylindrical elements without significant deformation, cumulatively giving a stent which is flexible along its length and about its longitudinal axis but which is still very stiff in the radial direction in order to resist collapse.
The stent embodying features of the invention can be readily delivered to the desired lumenal location by mounting it on an expandable member of a delivery catheter, for example a balloon, and passing the catheter-stent assembly through the body lumen to the implantation site. A variety of means for securing the stent to the expandable member on the catheter for delivery to the desired location are available. It is presently preferred to compress the stent onto the balloon. Other means to secure the stent to the balloon include providing ridges or collars on the inflatable member to restrain lateral movement, or using bioresorbable temporary adhesives.
The presently preferred structure for the expandable cylindrical elements which form the stents of the present invention generally have a circumferential undulating pattern, e.g. serpentine. The transverse cross-section of the undulating component of the cylindrical element is relatively small and preferably has an apect ratio of about two to one to about 0.5 to one (e.g., the ratio of the height to the width of an undulation). A one to one apect ratio has been found particularly suitable. The open reticulated structure of the stent allows for the perfusion of blood over a large portion of the arterial wall which can improve the healing and repair of a damaged arterial lining.
The radial expansion of the expandable cylinder deforms the undulating pattern thereof similar to changes in a waveform which result from decreasing the waveform""s amplitude and the frequency. Preferably, the undulating patterns of the individual cylindrical structures are in phase with each other in order to prevent the contraction of the stent along its length when it is expanded. The cylindrical structures of the stent are plastically deformed when expanded (except with NiTi alloys) so that the stent will remain in the expanded condition and, therefore, they must be sufficiently rigid when expanded to prevent the collapse thereof in use. With superelastic NiTi alloys, the expansion occurs when the stress of compression is removed so as to allow the phase transformation from austenite back to martensite and as a result the expansion of the stent.
The elongated elements or members which interconnect adjacent cylindrical elements should have a transverse cross-section similar to the transverse dimensions of the undulating components of the expandable cylindrical elements. The interconnecting elements may be formed in a unitary structure with the expandable cylindrical elements from the same intermediate product, such as a tubular element, or they may be formed independently and connected by suitable means, such as by welding or by mechanically securing the ends of the interconnecting elements to the ends of the expandable cylindrical elements. Preferably, all of the interconnecting elements of a stent are joined at either the peaks or the valleys of the undulating structure of the cylindrical elements which form the stent. In this manner there is no shortening of the stent upon expansion, when measured from the outermost ends of the interconnecting members connected to the cylindrical elements at opposite ends of the stent.
The number and location of elements interconnecting adjacent cylindrical elements can be varied in order to develop the desired longitudinal flexibility in the stent structure both in the unexpanded as well as the expanded condition. These properties are important to minimize alteration of the natural physiology of the body lumen into which the stent is implanted and to maintain the compliance of the body lumen which is internally supported by the stent. Generally, the greater the longitudinal flexibility of the stent, the easier and the more safely it can be delivered to the implantation site.
In a presently preferred embodiment of the invention the stent is conveniently and easily formed by coating stainless steel hypotubing with a material resistant to chemical etching, and then removing portions of the coating to expose portions of underlying tubing which are to be removed to develop the desired stent structure. The exposed portions of the tubing are removed by chemically etching from the tubing exterior leaving the coated portion of the tubing material in the desired pattern of the stent structure. The etching process develops smooth openings in the tubing wall without burrs or other artifacts which are characteristic of mechanical or laser machining processes in the small sized products contemplated. Moreover, a computer controlled laser patterning process to remove the chemical resistive coating makes photolithography technology adaptable to the manufacture of these small products. The forming of a mask in the extremely small sizes needed to make the small stents of the invention would be a most difficult task. A plurality of stents can be formed from one length of hypotubing by repeating the stent pattern and providing small webs or tabs to interconnect the stents. After the etching process, the stents can be separated by severing the small webs or tabs which connect them. The stents may further be electrochemically polished in an aqueous solution and treated, if desired by applying a biocompatible coating.
These and other advantages of the invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.